Conventionally, inorganic materials such as selenium, cadmium sulfide and zinc oxide are used as photoconductive materials of an electrophotographic photoconductor in the electrophotographic process. The above-mentioned electrophotographic process is one of the image forming processes, through which the surface of the photoconductor is charged uniformly in the dark to a predetermined polarity, for instance, by corona charge. The uniformly charged photoconductor is exposed to a light image to selectively dissipate the electrical charge of the exposed areas, so that a latent electrostatic image is formed on the photoconductor. The thus folded latent electrostatic image is developed into a visible image by a developer comprising a coloring agent such as a dye or pigment, and a binder agent such as a polymeric material.
Fundamental characteristics required for the photoconductor in such an electrophotographic process are: (1) chargeability to an appropriate potential in the dark, (2) minimum dissipation of electrical charge in the dark, and (3) rapid dissipation of electrical charge when exposed to light.
However, while the above-mentioned inorganic materials have many advantaged, they have several shortcomings from the viewpoint of practical use.
For instance, a selenium photoconductor, which is widely used as present, satisfies the above-mentioned requirements (1) to (3) completely, but it has the shortcomings that the manufacturing conditions are difficult and, accordingly, its production cost is high. In addition, it is difficult to work it into the form of a belt due to its poor flexibility, and it is so vulnerable to heat and mechanical shocks than it must be handled with the utmost care.
A cadmium sulfide photoconductor and a zinc oxide photoconductor can be easily obtained by dispersing cadmium sulfide particles and zinc oxide particles respectively in a binder resin, and coating the thus prepared coating liquid on a support. However, they are poor in mechanical properties, such as surface smoothness, hardness, tensile strength and wear resistance. Therefore, they cannot be used in the repeated operation, as they are.
To solve the problems of the inorganic materials, various electrophotographic photoconductors employing organic materials are proposed recently and some are still put to practical use. For example, there are known a photoconductor comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluorene-9-on, as disclosed in U.S. Pat. No. 3,484,237; a photoconductor prepared by sensitizing poly-N-vinylcarbazole with a pigment of pyrylium salt, as disclosed In Japanese Patent Publication 48-25658; a photoconductor comprising as the main component an organic pigment as disclosed in Japanese Laid-Open Patent Application 47-37543; a photoconductor comprising as the main component a eutectic crystal complex of a dye and a resin, as disclosed in Japanese Laid-Open Patent Application 47-10735; a photoconductor prepared by sensitizing a triphenylamine compound with a sensitizer pigment, as disclosed in U.S. Pat. No. 3,180,730; a photoconductor comprising an amine derivative as a charge transporting material, as disclosed in Japanese Laid-Open Patent Application 57-195254; a photoconductor comprising poly-N-vinylcarbazole and an amine derivative as charge transporting materials, as disclosed in Japanese Laid-Open Patent Application 58-1155; and a photoconductor comprising as a photoconductive material a polyfunctional tertiary amine compound, in particular, a benzidine compound, as disclosed In U.S. Pat. No. 3,265,496, Japanese Patent Publication 39-11546 and Japanese Laid-Open Patent Application 53-27033.
These electrophotographic photoconductors have their own excellent characteristics and considered to be valuable for practical use. With various requirements of the electrophotographic photoconductor In the electrophotographic process taken into consideration, however, the above-mentioned conventional electrophotographic photoconductors cannot always meet all the above-mentioned requirements.
Electrophotographic photoconductors which comprise carbonate-group-containing compounds as the photoconductive materials are disclosed In U.S. Pat. Nos. 4,801,517, 4,806,443 and 4,806,444, and Japanese Laid-Open Patent Applications Nos. 3-221522 and 4-11627. Each of the carbonate-group-containing compounds for use in the photoconductors is a polymeric compound, so that it is difficult to purify the carbonate group-containing compound by column chromatography, recrystallization, distillation or sublimation in order to obtain such a high purity as required for the photoconductive material. Therefore, the impurities cannot completely be removed from the above-mentioned photoconductor, so that all the requirements for the photoconductor cannot be satisfied.
There is known an electrophotographic photoconductor of which photoconductive layer is prepared in such a manner that a low-molecular photoconductive material is dissolved or dispersed in a binder resin solution to form a resin composition and forming the photoconductive layer by casting the above prepared resin composition. However, when the photoconductive layer is formed using a mixture of the low-molecular photoconductive material and the binder resin, as previouisly mentioned, the resin solution of the photoconductive material easily tends to cause gelation to become while opaque, and induces phase separation depending on the kind of binder resin to be employed. As a result, the uniform photoconductive layer cannot be obtained, which has an adverse effect on the electrostatic properties and the durability of the photoconductor.
Furthermore, as described in Japanese Laid-Open Patent Application 3-221522, there are the problems of the gelation of a photoconductive layer coating liquid, and partial crystallization and cracks of the obtained photoconductive layer when a single high-molecular conductive material is used to prepare a coating liquid for the photoconductive layer. According to the description in the aforementioned application, it is necessary to control the copolymerization ratio of the high-molecular photoconductive material and adjust the viscosity of the coating liquid for the photoconductive layer to solve the above-mentioned problems.
Furthermore, a dihydroxy compound serving as a raw material for preparation of a charge transporting material is conventionally known, as disclosed in Japanese Laid-Open Patent Applications 1-105260 and 3-294251 and U.S. Pat No. 4,801,517. A high-molecular charge transporting material is derived from the above-mentioned dihydroxy compound.
The following hydroxy compounds are conventionally known, o-(diphenylamino)phenol [Registry No. 25069-88-9], 3-(diphenylamino)phenol [Registry No. 107396-23-6], 4-(diphenylamino)phenol [Registry No. 25069-86-7], 3-{bis(4-methylphenyl)amino}phenol [Registry No. 80323-16-6], and 4-(diphenylamino)benzylalcohol [J. Polym. Sci. Polym. Chem. Ed., vol. 21, p. 969 (1983)].